Nodular regenerative hyperplasia (NRH) is a rare liver disease in which normal liver tissue transforms into multiple, small clusters (nodules) of replicating liver cells (regenerating hepatocytes). NRH often does not cause signs or symptoms (so is likely underdiagnosed), but in some people, it leads to a type of portal hypertension called non-cirrhotic portal hypertension (NCPH). NCPH refers to abnormally high blood pressure in the portal vein (the vein that carries blood from the intestine to the liver) that is not caused by cirrhosis (scarring of liver tissue that impairs its function). NCPH typically is less severe than portal hypertension caused by cirrhosis. Symptoms of portal hypertension may include fatigue, edema, itching, jaundice, and abdominal discomfort or swelling due to fluid build-up (ascites). Complications that may develop include bleeding esophageal varices and an enlarged and overactive spleen (hypersplenism). Liver function in most people with NRH remains normal (even in those who develop NCPH), and liver failure in people with NRH and NCPH is rare. The cause of NRH is not well-understood. It is thought that the liver's blood vessels somehow become damaged or inflamed, impairing blood flow to parts of the liver. This in turn may cause cells in adjacent parts to 'overcompensate' and replicate more than they should, leading to NRH. While some people with NRH have no known underlying health problems, NRH is often associated with rheumatic, autoimmune, hematologic (blood-related), and myeloproliferative disorders; chronic or recurrent infections; immune deficiency; or exposure to certain medications or toxins. NRH may be diagnosed based on signs and symptoms (when present), liver function tests (which usually are normal), medical history (including drug or toxin exposure), imaging studies, and liver biopsy (to rule out cirrhosis).